Runway numbers are based on the magnetic heading of the runway, rounded off to the nearest ten degrees. Because the magnetic variation changes a bit (usually around 10-15 seconds of arc a year) every now and then, and usually because of the rounding over rules, the numbers need to change.

So, a couple times a year in the US, runways are closed for a day or two, the numbers painted over and then repainted. This is a big hassle if there is an instrument approach, since all of the approaches, charts, etc have to be re-surveyed and reprinted.